Nonlinear optics is concerned with the interactions of electromagnetic fields in various media to produce new fields altered in phase, frequency, amplitude, or other propagation characteristics from the incident fields. In order to gain an insight into the origin of nonlinear optical effects, the polarization P induced in a molecule by a local electric field E can be expressed by Equation 1: EQU P=.alpha.E+.beta.E.sup.2 +.gamma.E.sup.3 ( 1)
where
P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .alpha., .beta., and .gamma. are the first, second, and third order polarizabilities, each of which is a function of molecular properties. PA0 .beta. and .gamma. are also referred to as first and second hyperpolarizabilities, respectively. The molecular level terms of Equation 1 are first order or linear polarization .alpha.E, second order or first nonlinear polarization .beta.E.sup.2, and third order or second nonlinear polarization .gamma.E.sup.3. PA0 P is the total induced polarization, PA0 E is the local electric field created by electromagnetic radiation, and PA0 .chi..sup.(1), .chi..sup.(2), and .chi..sup.(3) are the first, second, and third order polarization susceptibilities of the electromagnetic wave transmission medium. .chi..sup.(2) and .chi..sup.(3) are also referred to as the first and second nonlinear polarization susceptibilities, respectively, of the transmission medium. The macromolecular level terms of Equation 2 are first order or linear polarization .chi..sup.(1) E, second order or first nonlinear polarization .chi..sup.(2) E.sup.2, and third order or second nonlinear polarization .chi..sup.3 E.sup.3.
On a macromolecular level corresponding relationships can be expressed by Equation 2: EQU P=.chi..sup.(1) E+.chi..sup.(2) E.sup.2 +.chi..sup.(3) E.sup.3( 2)
where
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Engl. 23 (1984) 690-703, and Zyss "Nonlinear Organic Materials for Integrated Optics", Journal of Molecular Electronics, Vol. 1, pp. 22-45, 1985, disclose a variety of nonlinear optical end uses that can be served by utilizing .chi..sup.(2) or .chi..sup.(3) properties of a propagation medium.
Interest in nonlinear optical devices has particularly centered on devices relying on second order Polarization susceptibilities. To achieve on a macromolecular level second order polarization (.chi..sup.(2) E.sup.2) of any significant magnitude, it is essential that the transmission medium exhibit second order (first nonlinear) polarization susceptibilities, .chi..sup.(2), greater than 10.sup.-9 electrostatic units (esu). To realize such values of .chi..sup.(2) it is necessary that the first hyperpolarizability .beta. be greater than 10.sup.-30 esu.
A significant difficulty encountered in finding suitable molecular dipoles for second order polarization effects lies in the molecular requirements that must be satisfied to achieve usefully large values of .beta.. For a molecule to exhibit values of .beta. greater than zero, it is necessary that the molecule be asymmetrical about its center--that is, noncentrosymmetric. Further, the molecule must be capable of oscillating (i.e., resonating) between an excited state and a ground state differing in polarity. It has been observed experimentally and explained by theory that large .beta. values are the result of large differences between ground and excited state dipole moments as well as large oscillator strengths (i.e., large charge transfer resonance efficiencies).
For .chi..sup.(2) to exhibit a usefully large value it is not only necessary that .beta. be large, but, in addition, the molecular dipoles must be aligned so as to lack inversion symmetry. The largest values of .chi..sup.(2) are realized when the molecular dipoles are arranged in polar alignment--e.g., the alignment obtained when molecular dipoles are placed in an electric field.
Second order polarization (.chi..sup.(2) E.sup.2) has been suggested to be useful for a variety of purposes, including optical rectification (converting electromagnetic radiation input into a DC output), generating an electro-optical (Pockels) effect (using combined electromagnetic radiation and DC inputs to alter during their application the refractive index of the medium), phase alteration of electromagnetic radiation, and parametric effects, most notably frequency doubling, also referred to as second harmonic generation (SHG).
For a number of years the materials employed for achieving second order polarization effects were noncentrosymmetric inorganic crystals, such as potassium dihydrogen phosphate and lithium niobate. Williams postulates mathematically and experimentally corroborates second order polarization susceptibilities in organic dipoles equalling and exceeding those of conventional inorganic dipoles.
A number of difficulties have been encountered in attempting to prepare efficient optical devices employing an organic layer for the nonlinear propagation of electromagnetic radiation. If optical transmission is attempted through the organic layer while its upper surface is in direct contact with an electrode or other electrical conductor, significant optical losses are incurred. An optically passive layer over the organic layer has been suggested to enhance transmission efficiency. Such arrangements are disclosed by Ulman et al. U.S. Pat. No. 4,792,208, for example.
There are several difficulties involved. First, common inorganic deposition techniques, such as sputtering, molecular beam epitaxy, chemical vapor deposition, and the like, produce comparatively thin layers that are optically inefficient in reducing electromagnetic energy losses.
While there are varied techniques available for the deposition of thicker organic protective overcoats, organic nonlinear optical propagation media are susceptible to degradation by overcoating by conventional organic overcoating techniques. High deposition temperatures are precluded by the thermal stability limitations of organic nonlinear propagation media. Solvent coatings onto organic nonlinear optical propagation media can disturb the molecular alignment within the organic propagation layer, particularly in those forms requiring molecular alignment. Also, the organic layer is susceptible to degradation by dissolution in the coating solvent. Further, the organic propagation media and overcoating materials can crystallize as solvent is removed, leading to radiation scattering on transmission.
J. I. Thackera, G. F. Lipscomb, M. A. Stiller, A. J. Ticknor, and R. Lytel, "Poled Electro-Optic Waveguide Formation in Thin-Film Organic Media", Appl. Phys. Lett. 52 (13), Mar. 28, 1988, pp. 1031-1033, and, by the same authors, "Organic Electro-Optic Waveguide Modulators and Switches" SPIE Vol. 971 Nonlinear Optical Properties of Organic Materials (1988), pp. 218-229, are examples of attempt organic overcoats in combination with organic nonlinear optical propagation layers.